smtp.sarenet.es has no MX records.
Question and response
Question
dig smtp.sarenet.es. MX
Answer
The response has no answer records.
Authority
sarenet.es. 300 SOA ns1.sarenet.es. dnsmaster.sarenet.es. 2026030701 86400 7200 2592000 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0